The present invention concerns a tool for pressure shaping of metal plate and intended for use in a conventional hydraulic or mechanical press with at least one die, the tool comprising a shimming plate to be disposed between a tool part and further pressing tools, where the shimming plate is provided with at least one chamber formed in the shimming plate at at least one side as seen in the direction of pressing and which inwards is delimited by the shimming plate and outwards is delimited by a flexible membrane fastened to an edge area of the chamber, which least one chamber is provided along a given length outward from the die and extending outward from and around this, and where in the shimming plate there is formed access to the chamber for a fluid for creating a pressure in the chamber, and where the metal plate to be shaped is intended to be placed between the further pressing tools, alternatively between one of the further pressing tools and the shimming plate.
U.S. Pat. No. 3,420,089 describes a way of individually regulating the pressure between draw rings and a blank. A tool comprises a number of annular chambers each extending all the way around a die and which are mutually separated in direction away from the die. Each of the annular chambers is provided with inlets for hydraulic oil, and the pressure from the hydraulic oil may be regulated individually for each of the chambers. The tool is intended to create a uniform, though individual, pressure everywhere around the periphery of the die within each the chambers. Thereby the surface pressure can be regulated so that it is least at the outermost part of the periphery and increases in direction inwards toward the innermost part of the periphery around the die.
This tool, however, implies some disadvantages. The tool is suitable for radially symmetrical work pieces. For work pieces that are not radially symmetrical, it will not be possible to use the work piece and achieve a satisfactory result as there is no possibility of adjusting the surface pressure along the periphery with respect to the parameters influencing the finished item.
Furthermore, abrupt and relatively large jumps in the pressure distribution as seen in radial direction will occur on the blank to be shaped when an outer rim of the blank is drawn from the outer periphery and radially inward from one annular chamber to the other annular chamber. This may influence the shaping process itself and may result in uneven and sudden displacement of the die and consequently unsatisfactory shaping with the risk of great variations in the thickness of the finished item or, in the worst case, with the risk of breakage in the finished item.
EP 835 699 describes another way of regulating the pressure individually between blank holder rings and blank. The tool comprises a tool base plate provided with a number of holes which constitute guides for a number of lesser press mandrels. A multiple point pressure plate comprises a corresponding number of lesser hydraulic press pistons which are capable of acting on each single press mandrel extending through the guides in the tool base plate. By shaping the press mandrels individually and by acting individually on press mandrels by the lesser press pistons it is possible to regulate the pressure and the press surface individually at different places on the blank.
This tool implies a greater possibility of regulating the surface pressure also for blanks not being radially symmetrical. However, achieving the desired pressure at a given place on the blank is connected with extensive replacement of press mandrels and regulation of the pressure for each of the press mandrels. Furthermore, the tool implies the limitation that the position of the press pistons is determined by the multiple point pressure plate, and the shape of the press mandrels is limited by certain simple geometries. This means that it is not possible to achieve a completely optimal, individual adjusting of the pressure depending of the work piece to be shaped and depending on the shape of the blank.
It is the purpose of the present invention to provide a tool for press where it is possible to individually regulate the surface pressure at any place on the blank and over any area extending laterally around the die and away from the die under consideration of the parameters having influence on the finished item, so that the surface pressure on the blank becomes optimal with respect to the blank, the finished item and the shaping process itself.
This purpose is achieved with a tool which is peculiar in that a division into several chambers has been made in direction around the die, and that the chambers are formed by millings in the shimming plate, that a membrane is fastened to the edge of the milling in such way that the upper side of the membrane lies in the same plane as the surface of the shimming plate, that the milling has a depth, and that the membrane has a thickness less than or equal to the depth of the milling.
Provided chambers are formed with individually adjusted shape both in direction around the die and in direction away from the die, and where the number of chambers as seen in direction away from the die may be chosen individually, there is possibility of performing complete adjustment of the surface pressure on the blank under consideration of the different parameters having influence on the finished item. Furthermore, the shape of the chambers with the membrane placed in the milling will cause the upper side of the membrane to lie in the same plane as the surface of the shimming plate and make possible adjustment of the blank holder pressure as the membrane by pressurisation of the chambers is displaced out of contact with the bottom of the milling. Thus, in principle, a largely perfect item can be attained as the different parameters are completely adjusted to the blank concerned. In the remaining part of the present description, the term blank is used as designation for the metal plate constituting the starting item without limitation to the possible dimensions of a metal plate to be shaped by the tool and by the method according to the invention.
Adjustment takes place both statically and dynamically. The static adjustment is performed by the selected shapes and positions of the chambers around the die, and the dynamic adjustment takes place by regulating the pressure in the chambers during the shaping process itself. The static adjustment may be performed based on empirical data for the material and dimensions of the blank, for the lubrication used during the shaping process, and for the geometry and dimensions of the finished item. The dynamic adjustment may be performed based on running data about the pressing force and the pressing speed from the press used and data concerning the current pressure in each chamber in the shimming plate.
The chambers may be formed by real millings in the tool made in the shimming plate in order to form the chambers in the shimming plate and under the membrane when the membrane is attached to the shimming plate. At the location of the chambers it is hereby possible both to increase and to reduce the pressure between the shimming plate and the blank holder ring, alternatively the draw ring, and thereby to increase or to lessen the pressure on the metal plate to be shaped. When a volume of pressure medium completely corresponding to the volume of the chamber is supplied when the membrane is in the neutral state, i.e. in completely plane state, then the pressure will be the same as the pressure over the rest of the metal plate between the blank holder ring and the draw ring where the membrane contacts the blank holder ring, alternatively the draw ring.
The pressure is increased as compared with the pressure between the draw ring and the blank holder ring by the chamber being supplied a greater volume of pressure medium than the volume of the chamber which is delimited by bottom and sides of the milling and by the under side of the membrane in neutral state. The pressure is reduced as compared with the pressure between the draw ring and the blank holder ring by the chamber being supplied a lesser volume of pressure medium than the volume of the chamber which is delimited by bottom and sides of the milling and by the under side of the membrane in neutral state.
A method according to the invention is peculiar in that in a first step there is established a first pressure in a first zone between a draw ring and a blank holder ring and a second pressure in a number of chambers in a second zone, and that in a second, subsequent step there is established at least a second first pressure in the chambers in the first zone and optionally a second second pressure in the chambers in the second zone.
Such a method ensures that the dynamic adjusting of the surface pressure on the blank is optimal all the time in order hereby to reduce or to completely eliminate the risk of defects in the finished item. A current supervision of the pressure in each zone implies that it is possible immediately to adjust the pressure in one or more of the zones for obtaining a desired surface pressure on the blank during different steps of the shaping process and/or to compensate for irregularities during the shaping process.
The invention will then be described in more detail with reference to the accompanying drawing, where: